Sleep apnea, also spelled sleep apnoea, is a sleep disorder in which pauses in breathing or periods of shallow breathing during sleep occur more often than normal. Each pause can last for a few seconds to a few minutes and they happen many times a night. In the most common form, this follows loud snoring. A choking or snorting sound may occur as breathing resumes. Because the disorder disrupts normal sleep, those affected may experience sleepiness or feel tired during the day. In children, it may cause hyperactivity or problems in school.

Sleep breathing disorder
Other names	Sleep apnoea, sleep apnea syndrome
Obstructive sleep apnea
Pronunciation	/æpˈniːə/, /ˈæpniə/
Specialty	Otorhinolaryngology, sleep medicine
Symptoms	Pauses breathing or periods of shallow breathing during sleep, snoring, tired during the day
Complications	Heart attack, Cardiac arrest, stroke, diabetes, heart failure, irregular heartbeat, obesity, motor vehicle collisions, Alzheimer's disease, and premature death
Usual onset	Varies; 50% of women age 20–70
Types	Obstructive sleep apnea (OSA), central sleep apnea (CSA), mixed sleep apnea
Risk factors	Overweight, family history, allergies, enlarged tonsils, asthma
Diagnostic method	Overnight sleep study
Treatment	Lifestyle changes, mouthpieces, breathing devices, surgery
Frequency	~ 1 in every 10 people, 2:1 ratio of men to women, aging and obesity higher risk

Sleep apnea may be either obstructive sleep apnea (OSA), in which breathing is interrupted by a blockage of air flow, central sleep apnea (CSA), in which regular unconscious breath simply stops, or a combination of the two. OSA is the most common form. OSA has four key contributors; these include a narrow, crowded, or collapsible upper airway, an ineffective pharyngeal dilator muscle function during sleep, airway narrowing during sleep, and unstable control of breathing (high loop gain). It is often a chronic condition. Other risk factors include being overweight, a family history of the condition, allergies, and enlarged tonsils. Some people with sleep apnea are unaware they have the condition. In many cases it is first observed by a family member. Sleep apnea is often diagnosed with an overnight sleep study. For a diagnosis of sleep apnea, more than five episodes per hour must occur.

In CSA, the basic neurological controls for breathing rate malfunction and fail to give the signal to inhale, causing the individual to miss one or more cycles of breathing. If the pause in breathing is long enough, the percentage of oxygen in the circulation can drop to a lower than normal level (hypoxaemia) and the concentration of carbon dioxide can build to a higher than normal level (hypercapnia). In turn, these conditions of hypoxia and hypercapnia will trigger additional effects on the body (such as Cheyne-Stokes Respiration). Brain cells need constant oxygen to live, and if the level of blood oxygen goes low enough for long enough, brain damage and even death can occur. A systemic disorder, sleep apnea is associated with a wide array of effects, including increased risk of car accidents, hypertension, cardiovascular disease, myocardial infarction, stroke, atrial fibrillation, insulin resistance, higher incidence of cancer, and neurodegeneration. The exact effects of the condition depend on how severe the apnea is and on the individual characteristics of the person having the apnea.

Treatment may include lifestyle changes, mouthpieces, breathing devices, and surgery. Effective lifestyle changes may include avoiding alcohol, losing weight, stopping smoking, and sleeping on one's side. Breathing devices include the use of a CPAP machine. With proper use, CPAP improves outcomes. Evidence suggests that CPAP may improve sensitivity to insulin, blood pressure, and sleepiness. Long term compliance, however, is an issue with more than half of people not appropriately using the device. In 2017, only 15% of potential patients in developed countries used CPAP machines, while in developing countries well under 1% of potential patients used CPAP. Without treatment, sleep apnea may increase the risk of heart attack, stroke, diabetes, heart failure, irregular heartbeat, obesity, and motor vehicle collisions.

Alzheimer's disease and severe obstructive sleep apnea are connected because there is an increase in the protein beta-amyloid as well as white-matter damage. These are the main indicators of Alzheimer's, which in this case comes from the lack of proper rest or poorer sleep efficiency resulting in neurodegeneration. Having sleep apnea in mid-life brings a higher likelihood of developing Alzheimer's in older age, and if one has Alzheimer's then one is also more likely to have sleep apnea. This is demonstrated by cases of sleep apnea even being misdiagnosed as dementia. With the use of treatment through CPAP, there is a reversible risk factor in terms of the amyloid proteins. This usually restores brain structure and diminishes cognitive impairment.

OSA is a common sleep disorder. A large analysis in 2019 of the estimated prevalence of OSA found that OSA affects 936 million—1 billion people between the ages of 30–69 globally, or roughly every 1 in 10 people, and up to 30% of the elderly. Sleep apnea is somewhat more common in men than women, roughly a 2:1 ratio of men to women, and in general more people are likely to have it with older age and obesity.